SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    339432, 339433, 339438, 339434, 339439, 
    339445, 262432, 339440, 339441, 264700, 
    339446, 339447, 339449, 339689, 339448, 
    339450, 339435, 339442, 339443, 339436, 
    339451, 339437, 339444, 339476, 339456, 
    339550, 339461, 339454, 339547, 339471, 
    339545, 339474, 339548, 339470, 339544, 
    339455, 339543, 339549, 339698, 339472, 
    339542, 339487, 339480, 339475, 339688, 
    339546, 339687, 339690, 339462, 339700, 
    339811, 339459, 339679, 339473, 339813, 
    339678, 339465, 339457, 339477, 339541, 
    339489, 339699, 339469, 339458, 339482, 
    339685, 339468, 339453, 339730, 339731, 
    339478, 339463, 339709, 339467, 339460, 
    339808, 339464, 339806, 339810, 339701, 
    339807, 339479, 339452, 339729, 339485, 
    339809, 339812, 339484, 339483, 339466, 
    339486, 339488, 339481
  ) 
GROUP BY 
  p.product_id

Query time 0.00102

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (339432,339433,339438,339434,339439,339445,262432,339440,339441,264700,339446,339447,339449,339689,339448,339450,339435,339442,339443,339436,339451,339437,339444,339476,339456,339550,339461,339454,339547,339471,339545,339474,339548,339470,339544,339455,339543,339549,339698,339472,339542,339487,339480,339475,339688,339546,339687,339690,339462,339700,339811,339459,339679,339473,339813,339678,339465,339457,339477,339541,339489,339699,339469,339458,339482,339685,339468,339453,339730,339731,339478,339463,339709,339467,339460,339808,339464,339806,339810,339701,339807,339479,339452,339729,339485,339809,339812,339484,339483,339466,339486,339488,339481)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}